The present invention generally relates to the treatment of contaminated liquid, such as wastewater. More particularly, the present invention relates to a method for removing contaminants from a liquid by use of a gas replenishment system that controls levels of dissolved gas in the liquid. The gas contains species capable of converting dissolved solids into carbon dioxide and suspended solids, which are more easily separated from the liquid than dissolved solids. The present invention also pre-treats the wastewater, if needed, to remove large particles, fats, grease, and physically emulsified oils. Membrane separation such as nano filtration or reverse osmosis is also used to remove non-biodegradable organic materials and inorganic ions.
Industrial wastewater contains various pollutants. Such pollutants are present as large particles (larger than one micron), charge stabilized colloidal particles (oil and water emulsions, etc.) and dissolved species such as sugar, proteins, or inorganic ions. Regulations require removal of most or all of such pollutants before wastewater discharge into a publicly owned treatment works (POTW).
Complex treatment methods have been designed to remove the wide variety of physical and chemical species. In one instance, membrane and separation processes are used. Micro-filtration is used to remove large particles, while ultra-filtration is used to remove colloids and proteins, and reverse osmosis is used to remove ions and small species. But, subsequent cleaning is expensive and often is inefficient as the membranes get fouled with various species present in water. Another process for removing large particles from water is coagulation with inorganic species, such as ferric ions, ferrous ions, or aluminum ions, to neutralize particle charge. Subsequent sedimentation can be achieved in clarifiers. But, this process is slow and consequently requires large tanks.
Colloidal materials and micro-molecules must be removed after the removal of larger particles. Biodegradation is a particularly popular way of removing such materials. A collection of active microorganisms that grow in anaerobic or aerobic tanks may be capable of metabolizing these biodegradable materials. Anaerobic degradation is generally efficient, but, it can take months or even years to destroy biodegradable organic pollutants. Aerobic degradation is actually faster and can be used to treat wastewater with much lower organic loads. Low organic loads have chemical gas demand (COD) and biological gas demand (BOD) not larger than around 2,000 parts per million (ppm).
Biological aerobic industrial wastewater treatment should use water having high or varying loads of organic materials. Low organic loads can upset the biological aerobic wastewater treatment process. Unfortunately, most industrial wastewater is rich in nutrients and continuity varies in composition.
Accordingly, there is a continuing need to provide an industrial wastewater treatment system incorporating aerobic biodegradation while accommodating the amount of nutrients in the industrial wastewater over time. The present invention fulfills these needs and provides further related advantages.